In recent years, there has been an increasing use of a glass-made optical element over an extensive range as a lens for digital camera, optical pickup lens for DVD and others, camera lens for mobile phones, coupling lens for optical communications, various types of mirrors and others.
In this connection, a molded glass product manufactured by press molding of a glass material with a molding die has come into more frequent use as the aforementioned glass-made optical element.
Various techniques have been proposed to manufacture the molded glass product to be used as an optical element. One of these techniques is the method of dropping the molten glass drop from a molten glass dropping nozzle to manufacture a molded glass product by direct press molding the molten glass drop using a molding die. This method is characterized by a high degree of productivity in principle, and is therefore attracting attention and is being studied with greater enthusiasm.
The method of manufacturing a molded glass product by press molding of the molten glass drop falling from the molten glass dropping nozzle (hereinafter abbreviated as “nozzle”) includes: (1) a method wherein the molten glass drop is separated from the molten glass dropping nozzle and is made to fall on the lower die, whereby press molding is performed by the upper and lower molding dies (e.g., Unexamined Japanese Patent Application Publication No. H1-308840); and (2) a method wherein molten glass equivalent to a plurality of drops is retained in the lower die without molten glass drop being separated from the molten glass dropping nozzle, and the molten glass is then separated, whereby press molding is performed by the upper and lower molding dies (e.g., Unexamined Japanese Patent Application Publication No. H6-206730).
Unexamined Japanese Patent Application Publication No. H1-308840 discloses that the mass of the molten glass drop falling from the molten glass dropping nozzle can be expressed by the following formula:mg=2πry  (Formula 1)
wherein m is the mass of a molten glass drop, g is the gravity acceleration, r is the radius of the front end of the molten glass dropping nozzle, and γ is the surface tension of molten glass.
According to the aforementioned Formula 1, the mass of the molten glass drop is proportional to the diameter of the front end of the molten glass dropping nozzle. Thus, an increase in the diameter of the front end of the molten glass dropping nozzle provides the molten glass drop having a larger mass. In actual practice, however, the Formula 1 is applicable in a limited range. Thus, there has been a limit in the mass of the molten glass drop obtained in the conventional art.
FIG. 1 is schematic diagrams showing the state of the molten glass supplied to the front end of the conventional molten glass dropping nozzle. Referring to FIG. 1, the following describes the limit of the mass of the molten glass drop when the conventional molten glass dropping nozzle is employed: FIG. 1(a) shows the nozzle 1a wherein the outer diameter φRa is comparatively small (about +10 mm or less). Molten glass is supplied to the front end through the flow path having a diameter of φra arranged inside the nozzle 1a. The molten glass drop 3a retained in the front end falls under its own weight when the molten glass drop 3a has reached to a predetermined level of mass. As described above, when there are molten glass and molten glass drop formed by wetting over an extensive range on the front end of the nozzle, the mass of the molten glass drop can be increased by increasing the outer diameter of the front end.
In the nozzle 1b of FIG. 1(b), in the meantime, both the outer diameter φRb of the nozzle and diameter φrb of the flow path are greater than the diameter of the nozzle 1a in order to get greater molten glass drops. However, if the diameter of the flow path is increased too much as in this case, there will be an increase in the flow rate of the molten glass flowing in the flow path. Thus, the molten glass flows out of the front end of the nozzle in a line, with the result that a molten glass drop is not formed on the front end.
In the nozzle 1c of FIG. 1(c), only the outer diameter φRb of the nozzle is increased, with the diameter of the flow path kept unchanged (φra). In this case, there is a smaller flow rate of the molten glass flowing through the flow path. This ensures the molten glass drop to fall from the front end without the molten glass running in a liquid line. However, unlike the case of the nozzle 1a, only the center portion of the front end of the nozzle gets wet with the molten glass. The molten glass does not cover the peripheral portion. Thus, the mass of the molten glass drop is only slightly different from that in the case of nozzle 1a. A further increase in the outer diameter of the front end does not increase the mass of the molten glass drop.
As described above, in the conventional molten glass dropping nozzle the mass of the molten glass drop can be increased by the outer diameter of the front end only when the outer diameter is comparatively small. The mass of the molten glass drop cannot be increased, even if the outer diameter is further increased. Accordingly, a large-sized molded glass product cannot be produced by the method described in Unexamined Japanese Patent Application Publication No. H1-308840. This has created a big problem in the conventional art.
In the method described in Unexamined Japanese Patent Application Publication No. H6-206730, a predetermined amount of molten glass must be retained in the shortest possible time in order to ensure the uniformity of molten glass. At the same time, to permit the molten glass to be separated, the interval of dropping longer than a predetermined time must be provided. For this purpose, the mass of the molten glass drop per drop must be increased. To produce a large-sized molded glass product, the mass of the molten glass drop obtained by the conventional nozzle is insufficient. A solution to this problem has been eagerly anticipated.
The object of the present invention is to solve the aforementioned problems, and to provide a molten glass dropping nozzle capable of allowing a large-sized molten glass drop to fall, and a molded glass product manufacturing method and manufacturing apparatus capable of manufacturing a large-sized molded glass product using the aforementioned molten glass dropping nozzle.